By combining a lens comprised of high refractive index, low dispersion glass with a lens comprised of an ultra-low dispersion glass, it is possible to achieve a compact optical system while correcting for chromatic aberration. Thus, such lenses perform an extremely important role as optical elements in image pickup optical systems and projecting optical systems such as projectors.
Japanese Unexamined Patent Publication (KOKAI) Showa No. 60-33229, which is expressly incorporated herein by reference in its entirety, discloses a high refractive index, low dispersion glass which, while not being an optical element material for image pickup optical systems or projecting optical systems, has a refractive index of 1.90 to 2.10 and an Abbé number νd of 22 to 35.
By the way, in high refractive index, low dispersion glasses, the quantity of glass network-forming components that contribute to stabilizing the glass should be decreased to increase the quantity of components that raise the refractive index. Thus, glass stability may decrease and there are cases where phenomena such as a tendency to devitrify occur in the manufacturing process.
Further, when the quantity of components that raise the refractive index is increased, the melting temperature should be raised to obtain a homogeneous glass. There are also problems in that materials making up the melting vessel, such as platinum, leach out into the glass melt and glass coloration becomes pronounced. Further, the thermal stability of the glass decreases and the glass tends to devitrify.